1. Field of the Invention
The present invention relates to a pressurized vessel containing certain high purity gases under pressure therein and wherein the inner surfaces of the vessel are made by rotation molding and made of specific chemical polymers.
2. Brief Description of Art
Liquid- and gas-containing vessels having their inner surfaces covered with a polymeric coating are well known. The inner polymer coating in these liquid vessels is used to prevent corrosion of the vessel walls by the particular liquids or gases contained therein.
One example of such vessel is described in U.S. Pat. No. 5,474,846, which issued on Dec. 12, 1995 with George A. Haldenby as the named inventor. This Haldenby patent teaches a steel cylinder for holding high purity gases which has been treated to provide a uniform polymeric coating on the internal surfaces of the cylinder. The patent explicitly discloses only polyamides and polyolefins such as polyethylene and polypropylene as suitable polymeric materials. The patent also mentions that the polymeric materials may be subjected to a fluorination step to further improve the barrier properties of the polymeric materials. There are disadvantages associated with the Haldenby vessel. The separate fluorination treatment increases the cost of the vessel and may introduce unwanted impurities into the vessel. Generally, only the top surface of the polymeric layer is treated and the rest of the layer is unaffected.
It has also been believed that certain high purity gases can easily penetrate polyethylene or polypropylene layers and cause corrosion to the underlying metal walls of the vessel. The products of such corrosion can then migrate back through the polyethylene or polypropylene layer and become unwanted impurities in those high purity gases.
Alternatively, as shown in Air Products Specialty Gas Cylinder Product Brochure, page 32, hydrogen bromide and hydrogen chloride gases have been stored in carbon steel or in the presence of TEFLON, KEL-F, or HASTELLOY B material to resist corrosion. However, many types of TEFLON, KEL-F or other polymeric materials cannot be used as internal coating layers on steel cylinders because they are not suitable for rotational molding application operations. Because of these problems with internal polymeric coatings in steel cylinders, sellers of high purity gases have recently turned to using special non-corrosive metal cylinders such as nickel with no internal coating layers. However, the cost of nickel cylinders and the like is much higher than ordinary steel cylinders. The present invention provides a solution to these problems without the high cost associated with special non-corrosive metal cylinders.